Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume, printing press in order to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy customer requirements. This can result in a large loss of profits. By utilizing pre-press color proofing time and money can be saved.
One such commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,268,708 to Harshbarger et al., is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media. This is accomplished by applying a sufficient amount of thermal energy to the dye donor material to form an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem (which includes a lathe bed scanning frame, translation drive, translation stage member, print-head, and vacuum imaging drum), and thermal print media and dye donor material exit transports.
The operation of the image processing apparatus comprises metering a length of the thermal print media (in roll form) from the material assembly or carousel. The thermal print media is then measured and cut into sheet form of the required length and transported to the vacuum imaging drum, registered, wrapped around and secured onto the vacuum imaging drum. Next a length of dye donor material (in roll form) is also metered out of the material supply assembly or carousel, then measured and cut into sheet form of the required length. It is then transported to and wrapped around the vacuum imaging drum, such that it is superposed in the desired registration with respect to the thermal print media (which has already been secured to the vacuum imaging drum).
After the dye donor material is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and the dye donor material on the spinning vacuum imaging drum while it is rotated past the printhead that will expose the thermal print media. The translation drive then traverses the printhead and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the dye donor material is then removed from the vacuum imaging drum. This is done without disturbing the thermal print media that is beneath it. The dye donor material is then transported out of the image processing apparatus by the dye donor material exit transport. Additional dye donor materials are sequentially superposed with the thermal print media on the vacuum imaging drum, then imaged onto the thermal print media as previously mentioned, until the intended image is completed. The completed image on the thermal print media is then unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport.
The scanning subsystem, or write engine, of the lathe bed scanning type comprises the mechanism that provides the mechanical actuators for imaging drum positioning and motion control to facilitate placement, loading onto, and removal of the thermal print media and the dye donor material from the vacuum imaging drum. The scanning subsystem, or write engine, provides the scanning function by retaining the thermal print media and dye donor material on the rotating vacuum imaging drum. This generates a once per revolution timing signal to the data path electronics as a clock signal while the translation drive traverses the translation stage member and printhead axially along the vacuum imaging drum in a coordinated motion with the vacuum imaging drum rotating past the printhead. This is done with positional accuracy maintained, to allow precise control of the placement of each pixel, in order to produce the intended image on the thermal print media.
The lathe bed scanning frame provides the structure to support the vacuum imaging drum and its rotational drive. The translation drive with the translation stage member and printhead are supported by the two translation bearing rods, which are substantially straight along their longitudinal axis and positioned parallel to the vacuum imaging drum and lead screw. Consequently, they are parallel to each other, forming a plane along with the vacuum imaging drum and lead screw.
The translation bearing rods are, in turn, supported by the outside walls of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine. The translation bearing rods are positioned and aligned therebetween, for permitting low friction movement of the translation stage member and the translation drive. The translation bearing rods are sufficiently rigid for this application, so as not to sag or distort between the mounting points at their ends. They are preferably as exactly parallel as is possible with the axis of the vacuum imaging drum. The front translation bearing rod is preferably arranged so that the axis of the printhead lies precisely on the axis of the vacuum imaging drum. The axis of the printhead is located perpendicular, vertical, and horizontal to the axis of the vacuum imaging drum. The translation stage member front bearing is arranged to form an inverted "V". The translation stage member, with the printhead mounted on the translation stage member, is preferably held in place only by its own weight. The rear translation bearing rod locates the translation stage member--with respect to rotation of the translation stage member--about the axis of the front translation bearing rod. This is done so that there is no over constraint of the translation stage member that might cause it to bind, chatter, or otherwise impart undesirable vibration to the translation drive or printhead during the writing process. Such vibrations can cause unacceptable artifacts in the intended image. This benefit is accomplished by the rear bearing, which engages the rear translation bearing rod on the diametrically opposite side of the translation bearing rod on a line that is perpendicular to a line connecting the centerlines of the front and rear translation bearing rods.
The translation drive is for permitting relative movement of the printhead by synchronizing the motion of the printhead and stage assembly such that the required movement is made smoothly and evenly throughout each rotation of the drum. A clock signal generated by a drum encoder provides the necessary reference signal accurately indicating the position of the drum. This coordinated motion results in the printhead tracing out a helical pattern around the periphery of the drum. The coordinated motion is accomplished by means of a DC servo motor and encoder which rotates a lead screw that is typically, aligned parallel with the axis of the vacuum imaging drum. The printhead is preferably placed on the translation stage member in a "V" shaped groove, which is formed in the translation stage member. The printhead is selectively locatable with respect to the translation stage member, thus it is positioned with respect to the vacuum imaging drum surface. By adjusting the distance between the printhead and the vacuum imaging drum surface, as well as angular position of the printhead about its axis using adjustment screws, an accurate means of adjustment for the printhead is provided. Extension springs provide the load against these two adjustment means. The translation stage member and printhead are attached to a rotatable lead screw (having a threaded shaft) by a drive nut and coupling. The coupling is arranged to accommodate misalignment of the drive nut and lead screw so that only rotational forces and forces parallel to the lead screw are imparted to the translation stage member by the lead screw and drive nut. The lead screw rests between two sides of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine, where it is supported by deep groove radial bearings. At the drive end the lead screw continues through the deep groove radial bearing, through a pair of spring retainers, that are separated and loaded by a compression spring to provide axial loading, and to a DC servo drive motor and encoder. The DC servo drive motor induces rotation to the lead screw moving the translation stage member and printhead along the threaded shaft as the lead screw is rotated. The lateral directional movement of the printhead is controlled by switching the direction of rotation of the DC servo drive motor and thus the lead screw.
The printhead includes a plurality of laser diodes which are coupled to the printhead by fiber optic cables which can be individually modulated to supply energy to selected areas of the thermal print media in accordance with an information signal. The printhead of the image processing apparatus includes a plurality of optical fibers coupled to the laser diodes at one end and the other end to a fiber optic array within the printhead. The printhead is movable relative to the longitudinal axis of the vacuum imaging drum. The dye is transferred to the thermal print media as the radiation, transferred from the laser diodes by the optical fibers to the printhead and thus to the dye donor material and is converted to thermal energy in the dye donor material.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. As noted above, thermal print media is stored in roll form inside the apparatus and is metered and slit to length as needed. The cut edge requires a precision cut so that the media wraps closely about the vacuum drum. An imperfect cut can cause the media to seal improperly to the vacuum provided by the vacuum drum. Imperfectly cut media may even protrude slightly from the drum periphery. Since drum rotation is at high RPM (600 RPM and higher), this could result in loss of vacuum seal, which can cause fly-off of the media, loss of the print job in process, and even damage to equipment optics. Because the media is in the form of a polyester sheet, such as a film-base, cutting components must be carefully designed to prevent buckling or curling. These effects are known to be a problem in slitting sheets of such material.
Another drawback of the conventional approach to media sheet feed for such devices is caused by the requirement, inherent to the use of a vacuum drum, that the sheet wraps almost completely about the drum circumference. Regardless of the image size, the same size thermal media sheet must be loaded onto the vacuum drum. This adds cost and waste to the printing process. In order to allow imaging on a sheet of a different size, the media manufacturer must produce media having a different width. The imaging apparatus manufacturer must provide a different imaging drum that is dimensioned to handle a different paper size. This arrangement proves inflexible for manufacturers of imaging systems and their customers alike.
Yet another drawback of the conventional approach to media sheet feed is a result of the method used for writing an image onto thermal media that is loaded on a rotating vacuum drum. In media manufacture, as the plastic sheets are processed and coated, any variation in coating tends to be along the width of the roll, rather than the length. This is due, in part, to some stretching of the roll during processing. The polyester film base is pulled and stretched while it is being drawn. The coating process has a consistent variation in the widthwise direction while the roll is being coated. The lengthwise coating variations are related to the film transport, coating materials transport, and coating drying process. These variations are typically random and do not create as strong an error as the widthwise coating variations. For some processes, such as Gravure coating, the cylinder used will contribute a strong, once-per-cycle error signature. However, the variations across the cylinder will usually be more objectionable. This variation contributes to banding and streak artifacts in the printed image. The printhead of the apparatus is translated in the direction of the roll width as the media sheet rotates on the drum (rotating in the direction of the roll length). The image is written in a helical swath pattern, which runs very nearly parallel to the direction of roll length. Any banding that is detectable due to the writing operation tends to occur along and between swaths, in the same direction as banding due to roll coating variation. Thus, using conventional sheet feed, both the inherent roll coating characteristics and the writing pattern have an additive effect on banding and streaks in the image.
For apparatus that use an imaging drum, sheet feed from rolled media onto the imaging drum conventionally follows the roll direction. That is, the imaging drum acts as a "roll" with its axis parallel to the axis of any media supply roll. This is the case with thermal printers, such as those disclosed in U.S. Pat. No. 5,276,464, Kerr et al., issued Jan. 4, 1994. This is also true for numerous other imaging devices that employ an imaging drum or cylinder, such as inkjet printers.
It can be seen that there are inherent problems with conventional image processing apparatus and that there is, therefore, a need for solutions to overcome these problems. The present invention concerns an image processing apparatus in which a vacuum drum holds imaging media. On the vacuum drum, a sheet of receiver media is retained in position, with a sheet of donor media superposed over the receiver media. Donor and receiver media are provided on a carousel that stores individual rolls of receiver media, and color donor media. To load a sheet of media onto the imaging drum, the apparatus rotates the carousel into position for the intended media. The media is metered from the roll, cut to length, and fed into a receiving area for pickup by the vacuum drum. The media feed direction is parallel to the axis of the vacuum drum and the media roll width is provided in proper dimension for wrapping media about the drum circumference, thus allowing a variable length of media to be supplied to the drum.
In the apparatus of the present invention, media sheets are loaded onto the vacuum drum, where the sheet feed direction is parallel to the drum axis. Precision cutting of the media within the image processing apparatus is generally not needed, since cut edges of the media lie along the direction of high-speed drum rotation and are thus not likely to cause problems of fly-off if cut imperfectly. The present apparatus allows a user to load a media sheet of appropriate length for the image being printed in the apparatus, since media width is the dimension required for loading onto the drum circumference and maintaining a vacuum seal. Importantly, the apparatus of the present invention orients donor media perpendicular to the direction of writing swaths when loaded on the imaging drum. This minimizes the additive effects of donor variation and writing swath direction, thereby minimizing visible artifacts, such as banding and/or streaking, in the output image.